Pressurized fluidized bed combustors provide for firing electric power plants with coal in an economic and environmentally acceptable manner. Generally, pressurized fluidized beds can be installed in existing steam turbine plants to achieve better heat rates in repowering applications or in new power plants having both gas and steam turbines in a combined cycle.
In the basic fluidized bed combustor, granular particles are supported by an air distributor plate. The air introduced into the bed through the distributor plate causes the particles to go into suspension and circulate. In this condition, the bed material resembles a viscous fluid, or fluidized bed. As the air flow rate is increased, bubbles of air form in the bed, giving it the appearance of a violently boiling liquid. The granular particles then move turbulently to promote good gas-to-solids contact.
If the bed is heated and a fuel is introduced, combustion occurs, with the fluidizing air becoming the combustion air. The turbulent bed motion can produce high combustion efficiency and homogeneous bed temperatures. The bed can be operated adiabatically or with heat-exchanging tubes within the bed to control the bed temperature. In general, the coolant used with an in-bed heat exchanger is either air or water.
The fluidized, turbulent bed provides a bed-to-tube heat transfer coefficient higher than the coefficient encountered in conventional forced-convection heat exchangers, thereby reducing the size requirement of the heat exchanger. The homogeneous bed temperature simplifies heat exchanger design and eliminates the possibility of hot spots.
Since almost any type of fuel can be burned in a fluidized bed, it is often used to burn unconventional materials. Coal, oil shale, heavy oil, sludge, solid waste, and wood chips have been used for fuel in a fluidized bed.
Such flexibility has value with the use of coal since coal combustion in a fluidized bed is independent of coal ash content. The only limits that the coal ash places on the fluidized bed combustor is the operating temperature, which is limited by the ash fusion point to less than 1800.degree. F.
In the late 1950s, it was discovered that when coal was burned in a fluidized bed composed of limestone or dolomite particles, sulfur emissions are reduced via the reaction 2SO.sub.2 +2CaO+O.sub.2 T 2CaSO.sub.4. Therefore, a fluidized bed combustor offers an alternative to a conventional coal fired plant with a scrubber. The fluidized bed can burn coals of varying sulfur content while maintaining low sulfur emissions by varying the limestone or dolomite feed to maintain the proper calcium-to-sulfur molar ratio. Unlike a scrubber, no equipment modifications are necessary to accomplish this balance. The calcium-sulfur reaction kinetics restrict the bed temperature to a minimum of 1450.degree. F.
Fluidized bed combustors can be operated at atmospheric or at elevated air pressure to reduce unit size and produce other benefits. Although atmospheric fluidized bed (AFB) combustors have been in commercial use for years, particularly in Europe, the firing of a pressurized fluidized bed (PFB) combustor with coal is relatively recent.
At present, there are no commercial-scale PFB combustor power plants. The first large-scale PFB application in the United States dates back to the early 1940s with the advent of the fluid catalytic cracking process. The majority of the fluidized beds built since then have been used in the process industry. Some useful information relating to PFB coal combustion has been obtained from these years of experience, most notably in the areas of solids feeding and removal from a pressurized vessel, hot gas cleanup, and expansion turbine design. This information has resulted in commercially available equipment suitable for PFB combustion applications.
To obtain more specific data on coal combustion in a PFB, a number of test facilities have been built and operated, and a commercial-scale demonstration plant has been in the construction stage. Test facilities have provided extensive information on all design aspects of a PFB combustor system. PFB combustor operability has been demonstrated, and parametric investigations of the variables that affect the performance of the PFB combustion process have been conducted. Key design and operation relationships include:
Sulfur capture as a function of sorbent type, gas residence time and calcium-to-sulfur molar ratio PA1 Pressure drop as a function of fluidizing velocity and bed depth PA1 Gas residence time as a function of fluidizing velocity and bed depth PA1 Combustion efficiency as a function of gas residence time, bed temperature and the percent excess air
Test data have shown that a Ca/S ratio of 1.5 or greater captures at least 90 percent of sulfur, and that proper selection of process variables produce combustion efficiencies of 99 percent and higher without fines recycle. It has also been found that NO.sub.x emissions are relatively insensitive to process variables and are lower than the federal limit of 0.6 lb/106 BTU.
The solid waste from a coal-fired PFB combustor is readily handled and mainly contains sulfated calcium and inert coal ash. The amount of solid waste produced in a PFB combustor is less than that produced by a stack-gas scrubber.
Additional public information on the state of the PFB art is presented in a September 1980 Oak Ridge National Laboratory report ORNL/TM-7401 entitled State of the Art of Pressurized Fluidized Bed Combustion System by R. L. Graves and in a 1981 IEEE paper entitled Pressurized Fluidized Bed Combustion of Coal For Electric Power Generation--The AEP Approach.
For coal combustion, the PFB combustor has several advantages over the AFB combustor. Carbon utilization and sulfur-capture efficiency are increased and fewer nitrous oxides are emitted at the elevated PFB pressure.
Another advantage of a PFB combustor is that it can be used in a combined cycle power plant to yield overall cycle efficiencies much higher than AFB or pulverized coal-fired steam turbine units.
Another major advantage in pressurizing a fluidized bed coal combustion system resides in the fact that the furnace size and hence the number of coal feed points are inversely related to the furnace pressure. Thus, the furnace size and cost can be reduced markedly by pressurizing the unit. However, to supply pressurized air to the funace economically for a gas turbine must be driven by the high temperature, high pressure combustor effluent gases to develop the relatively large power needed to drive the combustor inlet air compressor. The use of a gas turbine enables the cycle efficiency to be increased with an elevated furnace pressure such as 10 atm in a combined gas turbine-steam cycle similar to that employed in many power plants in operation in the United States employing oil or gas as fuel. On the other hand, the problems presented by turbine corrosion, erosion, and deposition if coal is used as fuel have created development difficulties for such a system.
With this background of PFB technology, a challenge to continued progress is to conceptualize and implement a PFB combined cycle and plant system in which the advantages of PFB are harnessed efficiently within the present and prospective limits of gas cleanup technology as it relates to protection of hot turbine parts against corrosion, erosion and deposition.